This invention relates to organoperoxide-curing silicone rubber compositions. More particularly, this invention relates to a silicone rubber composition that yields silicone rubber whose durometer can be freely reduced without a concomitant reduction in the mechanical properties of the silicone rubber.
Organoperoxide-curing silicone rubber compositions cure rapidly when heated to yield cold-resistant and heat-resistant silicone rubber. A general tactic employed here consists of the admixture of SiH-containing organopolysiloxane into these silicone rubber compositions in order to inhibit discoloration during cure (Japanese Patent Application Laid Open Kokai! Numbers Sho 53-40050 40,050/1978! and Hei 4-300962 300,962/1992!), to improve the tear strength of the silicone rubber (Japanese Patent Application Laid Open Kokai! Number Sho 56-41252 41,252/1981!), to improve the fatigue resistance of the silicone rubber (Japanese Patent Application Laid Open Kokai! Number Sho 62-197454 197,454/1987!), and to reduce the surface tack of the silicone rubber (Japanese Patent Application Laid Open Kokai or Unexamined! Numbers Sho 63-130663 130,663/1988! and Hei 2-124977 124,977/1990!). In order to improve the described properties, however, organohydrogenpolysiloxane or organohydrogencyclosiloxane containing at least 3 SiH per molecule is typically used, which causes such properties in the silicone rubber as the durometer and tensile strength to increase in value. At the same time, with respect to the silicone rubber compositions used to form packings, gaskets, tubing, and the fixing and pressure rolls in copiers and printers, the durometer of the silicone rubbers afforded by these compositions must be rigorously controlled. The durometer can be adjusted downward, for example, by blending a non-crosslinking silicone oil into the precursor silicone rubber composition. Another method for adjusting the durometer downward consists of preparing silicone rubber with the desired durometer by blending two different silicone rubber compositions--one that forms silicone rubber with a durometer below the desired value and one that forms silicone rubber with a durometer above the desired value--in freely selected proportions
The former method requires that large amounts of non-crosslinking silicone oil be admixed to achieve a reduction in the durometer of the silicone rubber. This creates problems such as a reduction in the mechanical strength of the resulting silicone rubber and bleed-out of the silicone oil from the silicone rubber. The existence of these problems has caused the latter method to be the method in general use. This notwithstanding, the latter method requires the preliminary preparation of at least two different silicone rubber compositions and also requires skill to prepare a silicone rubber with the desired durometer without impairing the mechanical properties.
The object of the present invention is to provide a silicone rubber composition that yields silicone rubber whose durometer can be freely reduced as desired without a concomitant reduction in the mechanical properties of the silicone rubber.